1. Field of the Invention
The present invention relates to a semiconductor laser device capable of emitting visible or near-infrared light and suitable for a light source in bar code readers, pointers, optical pickups, optical measuring instruments and the like, and more specifically to a semiconductor laser device having a so-called inner stripe structure, wherein driving currents flow through a stripe-shaped opening provided in a reversed pn-junction plane which extends along one side of a current blocking layer (CBL).
2. Description of the Related Art
FIG. 1 illustrates a structure of conventional semiconductor laser device, which is disclosed and called SBR structure or SBR laser in Jpn. Pat. Appln. KOKAI Publications Nos. 62-200785 and 62-200786 and etc.
In FIG. 1, reference numeral 1 indicates a metallic electrode on the p side; 2, a cap layer of p-type GaAs formed to reduce the contact resistance; 3-6, a second clad layer of p-type InGaAlP; 7, an active layer of InGaP; 8, a first clad layer of n-type InGaAlP; 9, a substrate of n-type GaAs; 10, a metallic electrode on the n side; and 11, a current blocking layer (CBL) of n-type GaAs.
The current supplied through the metallic electrode 1 flows through the cap layer 2, clad layers 3 to 6, active layer 7, clad layer 8, and substrate 9 in this order, and drains away through the metallic electrode 10. This current-flow is confined within a slit-shaped opening X formed in the current blocking layer 11. Therefore, it is the active layer just under the opening X, that is, directly under the ridge 4 where the current generates light gain.
Lasing is achieved by cooperation of the above light gain, optical resonator, and an optical slab-waveguide constituted of the clad layers 3 to 6, active layer 7 and clad layer 8.
The following are the dimensions and carrier concentrations of the main constituents of the SBR laser. The thickness of the CBL 11 is about 1.5 .mu.m and the carrier concentration thereof is 3.times.10.sup.18 cm.sup.-3. The CBL has to be thicker than the hole diffusion length in it. Otherwise, electrons optically pumped in the CBL by the laser light compensate the depletion region of the reversed pn-junction and this junction eventually turns on (Photo-induced turn-on). Thickness of the active layer 7 is 0.02 to 0.08 .mu.m. The clad layer 4 forms a ridge, the cross-section of which is trapezoid-shaped and 3 to 6 .mu.m wide at the bottom. The total thickness of the clad layers 3 to 6 is 1 to 3 .mu.m, while that of the clad layers 5 to 6 is 0.15 to 0.4 .mu.m. The clad layer 8 is 1.0 to 1.5 .mu.m thick. The length of an optical resonator, which is composed of opposed facet surfaces E1 and E2 is 400 to 800 .mu.m. Though the current is effectively converted into laser light in the semiconductor laser device, still some part of it is changed into heat. The active layer re-absorbs the laser light, converts it such undesirable heat, and is heated up until it melts away at one or both of the facet surfaces E1 and E2. Thus, electrostatic discharges from an operator's body or work bench damage the laser device. So do spike currents generated when switching the power supply ON or OFF. Therefore, the care should be taken not to exceed, even momentarily, the laser device's maximum current rating.
EIAJ (Electronic Industries Association of Japan) shows a standard method to evaluate durability of semiconductor devices against these discharges and spike currents (surge currents), where the circuit shown in FIG. 2 is used.
The evaluating procedure is as follows. First, the mercury switch SW is set to the CHARGE position and the capacitor C of 200 pF is charged up to a voltage V.sub.c of any value set by the variable voltage source VR. Then, SW is turned to the DISCHARGE position so that the electric charge on C drains away through the tested semiconductor laser device LD. If this discharge does not give the LD any damage, that is, the LD shows the same characteristics as before, the voltage V.sub.c is set a little higher and the same procedure is repeated until the LD shows any damage. Surge-damaging-voltage V.sub.s, defined as the value of V.sub.c that gives the LD the first damage, is practically useful as an index for the robustness of the LD.
Though surge-damaging-voltage V.sub.s greatly depends upon the structure ofisemiconductor laser devices, it generally ranges fairly low voltage of 20 to 80 V. SBR lasers are not the exception. Therefore, SBR lasers as well as semiconductor laser devices with other structures have to be handled very carefully not to be damaged by unintentional surge currents.
The electric resistance of SBR lasers is virtually equal to the resistance R.sub.s of the ridge 4, the about =6 .OMEGA.. Therefore, the time constant of the discharge in the test circuit shows in FIG. 2 is about 200 pF.times.6 .OMEGA.=1.2 nsec. This means that the discharge accomplishes within several nanoseconds. In such a short time interval, the critical surge current I.sub.s that causes a fatal damage on a semiconductor laser is considered to be in inverse proportion to the duration .tau..sub.s of the current. In other words, a semiconductor laser device is assumed to be damaged at a critical charge q.sub.s =I.sub.S .tau..sub.S, which depends on the specific device structure. The above critical charge q.sub.s is estimated to be 4 to 16.times.10.sup.-9 C by using the above values V.sub.s =20 to 80 V and C=200 pF. The existence of q.sub.s and its values have been confirmed by driving with rectangular current pulses, which damage semiconductor laser devices when the charge included in one pulse is nearly equal to the value of q.sub.s estimated above.
In Jpn. Pat. Appln. KOKAI Publication No. 4-309278, it is claimed that increased electric capacity of semiconductor laser devices improves their durability against surge currents. However, even if the structure disclosed there is used, the capacity of the device can be increased only five times as large as that of conventional devices at most. This increments is generally not enough to improve the durability of the device against surge currents, because time width of spike or pulse is generally still so short that the damage of the device is still ruled by the amount of charge which passes through the device in one spike or pulse as explained above. Consequently, it is concluded that in order to improve the durability of semiconductor laser devices against surge currents drastically, it is necessary to provide discharging paths other than the path through the ridge 4.